Image forming apparatuses having a fixing device heating and fixing a toner image to a recording medium, for instance a sheet have been known. A ready temperature of the fixing device is set constantly and by a margin. Thus, even on a printing job to which a maximum load is applied, the fixing device maintains good fixing performance up to the final print. However, a quantity of heat at which the fixing device requires actually differs depending on job conditions such as a type of toner, the number of printing sheets, and a sheet size. For this reason, when the image forming apparatus carries out a job having a low load applied to the fixing device, the ready temperature may be excessive to impede energy saving. Further, when returning from a sleep state (power saving state) to a ready state, the image forming apparatus takes a long time to do a warmup for raising a heating temperature of the fixing device to a higher ready temperature than necessary.